Derivative And Credit-Related Financial Instruments (Summary Of Internally Classified Watch List Letters Of Credit) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivative And Credit-Related Financial Instruments [Abstract]
Total watch list standby and commercial letters of credit	$ 195	$ 243
As a percentage of total outstanding standby and commercial letters of credit	3.60%	4.40%

Total internally classified watch list standby and commercial letters of credit (generally consistent with regulatory defined special mention, substandard and doubtful).

No definition available.

Name:	cma_TotalInternallyClassifiedWatchListLettersOfCredit
Namespace Prefix:	cma_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant